The invention relates to the field of microelectronics. More specifically, it relates to a process for producing inductive microcomponents on a substrate, which itself can incorporate an integrated circuit.
The components may especially be used in radiofrequency applications, for example in the field of telecommunications.
The subject of the invention is more specifically a process for obtaining circuits having a markedly higher performance than existing components, especially as regards the value of the quality factor. The process forming the subject of the invention also limits the number of steps needed to produce such components and ensures good reproducibility of the characteristics of the components that it allows to be fabricated.
In document FR 2 791 470, the applicant has described a fabrication process for producing microinductors or microtransformers on top of a substrate, and especially on top of an integrated circuit. To summarize, this process consists in depositing a layer of material having a low relative permittivity and then etching this material at an aperture made in the hard mask, vertically above a pad for connection with the rest of the integrated circuit, so as to define an interconnection hole, or via.
After having deposited a resist on top of the hard mask, said resist is etched to form the channels defining the geometry of the turns of the inductive component. Thereafter, copper is deposited electrolytically on top of the connection pad and in the channels defined in the top resist.
Such a process has a number of drawbacks, among which may be noted essentially the fact that the electro-deposition step ensures both the formation of the turns of the inductive component and the filling of the via, allowing contact with the metal pad connected to the integrated circuit. Since these regions have different depths, it follows that the electrodeposition is carried out differently at the turns and at the via. Thus, certain irregularities are observed in the formation of the turns, these being prejudicial to good uniformity of the electrical performance of the inductive component.
Furthermore, during the step of etching the top resist, it is necessary to etch longer at the via, compared with the regions in which the channels intended to accommodate the turns are formed. This difference in etch depth causes the release of chemical compounds at the bottom of the via, thereby interfering with the subsequent copper electrodeposition operation.
One of the objectives of the invention is to alleviate these various drawbacks, and especially to make it possible to produce components which have dimensional characteristics that are as precise as possible, so as to give optimum electrical performance.
The invention therefore relates to a process for fabricating an electronic component. Such a component incorporates an inductive microcomponent, such as an inductor or a transformer, which is placed on top of a substrate and connected to this substrate by at least one metal pad.
In accordance with the invention, this process is one which comprises the following steps:
a) depositing a layer of material having a low relative permittivity on the substrate;
b) depositing a layer forming a hard mask;
c) forming an aperture in the hard mask vertically above the metal pads;
d) etching the layer of material having a low relative permittivity down to the metal pad, in order to form an interconnection hole or via;
e) depositing a layer forming a copper diffusion barrier;
f) depositing a copper primer layer;
g) depositing a protective mask and removing it from the bottom of the via;
h) depositing copper, electrolytically, in the via;
i) removing the rest of the protective mask;
j) depositing a top resist layer with a thickness similar to the thickness of the turns of the inductive microcomponent;
k) etching the top resin layer in order to form channels defining the geometry of the turns of the inductive microcomponent;
l) depositing copper electrolytically in the channels thus etched;
m) removing the rest of the top resist layer;
n) etching the copper primer layer between the copper turns; and
o) etching the copper-diffusion barrier layer between the turns of the inductive microcomponent.
Thus, the process according to the invention links together a number of steps which provide certain improvements over the processes of the prior art. It will be noted in particular that the copper electrodeposition takes place in two separate steps, namely, to begin with, a first step for filling the via, thereby allowing firstly copper to be grown up to level with the lower plane of the inductive microcomponent. In a second step, a copper electro-deposition process is carried out, thereby forming simultaneously the turns of the inductive component and the region of connection between the turns and the via already filled in during the prior deposition step.
Separating these two copper deposition steps in this way ensures homogeneity of this deposition, this being favorable to uniformity of the shape of the turns, and therefore to the quality of the electrical performance and the reproducibility of the process.
It will also be noted that this process can be used on various types of substrate. Thus, in a first family of applications, the process can be used on a semiconductor substrate and especially a substrate that has been functionalized beforehand in order to form an integrated circuit.
In other types of application, it may be a specific substrate, such as an amorphous substrate of the glass or quartz type, or more generally a substrate possessing electrical, optical or magnetic properties suitable for certain applications.
In practice, the material having a low relative permittivity which is deposited on the substrate, may be benzocyclobutene (BCB), or else, a similar material whose relative permittivity is typically less than 3.
In practice, the thickness of this layer of material having a low relative permittivity may be between 10 and 40 microns, preferably being about 20 microns.
The thickness of this layer defines substantially the distance between the inductive component and the substrate. This distance, combined with the relative permittivity of the material of this layer, defines the parasitic capacitance existing between the inductive component and the substrate, and it is highly desirable to minimize this capacitance.
In practice, the material used to form the hard mask on top of the BCB may be chosen from the group comprising: SiC, SiN, Si3N4, SiON, SiO2, SiOC, Y2O3, Cr, taken individually or in combination.
The properties of these materials include good compatibility with BCB, especially good adhesion as hard mask on the BCB surface. These materials have mechanical properties suitable for them to be used in masking. This avoids the appearance of excessively high stresses at the junction between the hard mask and the subjacent BCB layer. Moreover, by a judicious choice of these materials having the function of a hard mask for the purposes of etching the vias, high selectivity of the BCB etching compared with these materials is acquired, so as to avoid any underetching of the BCB, and thus to obtain the desired profiles without delamination.
This is because the stresses between the BCB and the hard mask could be transferred right to the substrate and cause possible fractures in the latter. Such phenomena owing to excessively high stresses are especially observed in the processes of the prior art, which use thick layers of certain metals to produce the hard mask on top of a BCB layer, with as consequence the risk of poor adhesion.
In practice, and especially when the hard mask is conducting, and typically based on chromium, this hard mask may be removed before the copper-diffusion barrier layer is deposited, so as to remove any inter-turn conducting region.
According to another feature of the invention, a layer forming a copper diffusion barrier is deposited on top of the layer of material having a low relative permittivity, when the hard mask has been removed. This barrier layer allows the subjacent layer to be isolated from the copper that will be deposited subsequently, especially in the form of the primer layer. This characteristic barrier layer prevents the migration of copper through the layer of low relative permittivity, something which would have the effect of increasing this permittivity, and therefore of increasing the parasitic capacitance between the inductive microcomponent and the substrate, and of creating sources of defects. This barrier layer also prevents the copper from migrating into the substrate, which would have prejudicial consequences on the quality or the operation of the integrated circuit.
In practice, the barrier layer may be made of tungsten or from a material chosen from the group comprising: TiW, Ti, TiN, Ta, TaN, WN, Re, Cr, Os, Mo, Ru. These materials may be used individually or in combination.
Advantageously, in practice, the thickness of the copper-diffusion barrier layer may be between 100 and 400 xc3x85.
According to another feature of the invention, the process may include a step of enriching the copper primer layer. This primer layer acts as the electrode for the subsequent copper electrodeposition operations.
It may prove useful under certain conditions to improve the regularity and the morphology, the oxidation state of the copper, the roughness and the lack of nucleation sites in the primer layer. This primer layer is deposited by a physico-chemical technique, more particularly by the technique called sputtering and its ionized metal plasma variant. In this case, a step to enrich this primer layer by exposing the primer layer to an electrolyte solution is carried out. This solution, containing copper salts, allows copper to be deposited in any spaces existing between the copper islands deposited beforehand during the production of the primer layer, this enriching step therefore smoothing out this primer layer so as to improve the subsequent electro-deposition.
Advantageously, in practice, an annealing step may be carried out so as to increase the size of the copper crystals deposited during the electrodeposition steps. This annealing step, typically carried out by exposing the component to a temperature between 150 and 400xc2x0 C. for a time of a few minutes, ensures crystalline uniformity of the copper deposited, and therefore the homogeneity and the conducting nature of the copper which will form the turns of the inductive component. Thus, the electrical properties of the component are improved by reducing the number of singularities that could be the source of resistive spots or points of mechanical weakness.
Advantageously, in practice, a step of decontaminating the copper liable to migrate into the substrate, especially at the lateral and rear faces of the substrate, as well as around its circumference, may be carried out. This is because when the component is exposed to a solution containing copper salts soluble in a judiciously chosen solvent, it is necessary to remove any excess copper deposited. In fact, when this metal is deposited using electrolytic techniques and with a specific current distribution between the cathode and the anode, excess copper is generally observed to be deposited around the circumference of the substrate. Moreover, the convection and mass-transfer process, which is at the basis of the technique of depositing the element copper by electrolysis, generates, on the lateral or rear faces of the substrate, a possible flux and diffusion over certain regions of the substrate. To avoid any possible migration into the substrate, it is recommended to use this step.
In practice, this decontamination step may be performed after both of the two electrodeposition steps.
In practice, the protective mask deposited during the step following deposition of copper primer layers may be formed from a negative photoresist. This allows it to be easily removed at the bottom of the via in which the first copper electrodeposition will subsequently take place. Modifying the properties of the photoresist allows it to cure in the regions exposed during exposure of the lithography mask. A surface deposition of copper is thus avoided, by virtue of the screen thus formed on the surface of the enriched primer layer by the cured resist.
Advantageously, in practice, before the step of depositing the top resist, it is possible to carry out a treatment either with hexamethyldisilazane (HMDS) or divinyltetramethyldisilazane (DVTMDS), as desired. This treatment makes it possible to obtain good copper-resist adhesion properties, thereby improving the growth of the copper on the vertical sidewalls of the channels intended to accommodate the turns.
According to other features of the invention, a number of cleaning steps may be carried out using a chemical not corrosive to copper. These cleaning steps may be carried out after the copper electrodeposition, and after the step of depositing the copper primer layer, or else after the copper-diffusion barrier layer has been deposited.
The invention also relates to an electronic microcomponent that can be produced using the above-mentioned process. Such a component incorporates an inductive microcomponent placed on a substrate and connected to the latter by at least one metal pad.
This component comprises:
a layer of material having a low relative permittivity, lying on the top face of the substrate;
a number of metal turns defined on top of the layer of material having a low relative permittivity; and
a copper-diffusion barrier layer interposed between the metal turns and the layer of material having a low relative permittivity.